I'm facing problem in the oscillator design by using the ADS. I'm trying to design an fix frequency oscillator (colpitts) working at 90Mhz.
In in the initial stage , I set my biasing current at 10m and Vc at 5V for the transistor 2n3904. Then i try to make the amplifier to perform at unstable region which having S(1,1) >1 and S(2,2) >1 (thats the requirement for oscillator design) by selecting suitable load resistance. Pls look at attachments "S-result".

So the Z1 now is having negative resistance = -8.638 + 0.551j (normalised) . So i use thumb rule method where |R1|/3 = Rs And Xsj = - X1j.
Zs = 143.9 - 27.5j.
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The problem i facing is when i measured at output, the oscillator was not sustain oscillating. (result on attachments)